France is to reinstate controls on its borders - normally open to other countries in Europe's free-travel zone - for the period around a major UN climate conference in Paris.

Authorities are on alert for violent protesters as well as potential terror attacks around the November 30-December 11 conference.

Some 80 heads of state and tens of thousands of other people, are expected in Paris for the conference opening.

Interior minister Bernard Cazeneuve said that the controls will be in place for a month "in the context of terrorist threats that could come and stain this large international gathering that is carrying a grand message for humanity".

Europe's so-called Schengen zone of countries with open borders allows for occasional reintroduction of internal border checks, which some countries have done amid this year's migrant crisis.

France submitted a note to European Union authorities last month announcing border checks at all airports with international flights and at 131 land crossings with Belgium, Germany, Spain, Italy, Switzerland and Luxembourg, according to an EU document.

The French note cites Schengen rules allowing controls "when there is a serious threat to public policy".

France is still reeling from deadly attacks by Islamic extremists in January on satirical newspaper Charlie Hebdo and a kosher supermarket.

Since then, the country has seen several other smaller attacks or attempts, including when a heavily-armed Islamic radical was prevented by young American passengers from attacking a high-speed train in August.

France is also bombing Islamic State (IS) targets in Syria and Iraq and has troops fighting extremists in Africa.

The climate conference is aimed at reaching the most ambitious accord to date for world governments to reduce emissions that cause global warming.

Organisers expect at least 40,000 people in addition to tens of thousands of activists from environmental, human rights and other groups from around the world.

A big march is planned through Paris on November 29, and protest groups plan to try to blockade the conference site on December 11 and 12, as well as several other smaller-scale actions.

France faces routine protests that are largely peaceful but sometimes degenerate into violence by an extremist fringe.

The country saw particularly violent protests during a Nato summit in Strasbourg in 2009, when members of the violence-prone "black bloc" attacked police and set a hotel and customs station ablaze, leaving many injured and disrupting official meetings.

The last time France reinstated national border controls was for a Group of 20 summit in Cannes in 2011.